How to get the max and min value of a certain NodeSocket's slider?
These are blender properties 

https://docs.blender.org/api/current/bpy.props.html  In defining, for example a float type property bpy.props.FloatProperty can set its hard and soft minimums and maximums.  

A soft setting is the first range you force the property past up till the hard setting.

As an example using the Principled BSDF node 

&gt;&gt;&gt; node_tree = D.materials['Material'].node_tree
&gt;&gt;&gt; node = node_tree.nodes['Principled BSDF']


Each inputs default_value property settings can be found via input.bl_rna.properties["default_value"]

Loop over the enumerated inputs of the node and output the index, name, mins, and maxes of their default_value property.

&gt;&gt;&gt; for i, inp in enumerate(node.inputs):
...     i, inp.name
...     p = inp.bl_rna.properties["default_value"]
...     p.hard_min, p.soft_min, p.soft_max, p.hard_max
...     
(0, 'Base Color')
(0.0, 0.0, 1.0, 3.4028234663852886e+38)
(1, 'Subsurface')
(0.0, 0.0, 1.0, 1.0)
(2, 'Subsurface Radius')
(-3.4028234663852886e+38, -10000.0, 10000.0, 3.4028234663852886e+38)
(3, 'Subsurface Color')
(0.0, 0.0, 1.0, 3.4028234663852886e+38)
(4, 'Metallic')
(0.0, 0.0, 1.0, 1.0)
(5, 'Specular')
(0.0, 0.0, 1.0, 1.0)
(6, 'Specular Tint')
(0.0, 0.0, 1.0, 1.0)
(7, 'Roughness')
(0.0, 0.0, 1.0, 1.0)
(8, 'Anisotropic')
(0.0, 0.0, 1.0, 1.0)
(9, 'Anisotropic Rotation')
(0.0, 0.0, 1.0, 1.0)
(10, 'Sheen')
(0.0, 0.0, 1.0, 1.0)
(11, 'Sheen Tint')
(0.0, 0.0, 1.0, 1.0)
(12, 'Clearcoat')
(0.0, 0.0, 1.0, 1.0)
(13, 'Clearcoat Roughness')
(0.0, 0.0, 1.0, 1.0)
(14, 'IOR')
(-3.4028234663852886e+38, -10000.0, 10000.0, 3.4028234663852886e+38)
(15, 'Transmission')
(0.0, 0.0, 1.0, 1.0)
(16, 'Transmission Roughness')
(0.0, 0.0, 1.0, 1.0)
(17, 'Emission')
(0.0, 0.0, 1.0, 3.4028234663852886e+38)
(18, 'Alpha')
(0.0, 0.0, 1.0, 1.0)
(19, 'Normal')
(-3.4028234663852886e+38, -10000.0, 10000.0, 3.4028234663852886e+38)
(20, 'Clearcoat Normal')
(-3.4028234663852886e+38, -10000.0, 10000.0, 3.4028234663852886e+38)
(21, 'Tangent')
(-3.4028234663852886e+38, -10000.0, 10000.0, 3.4028234663852886e+38)


Note a property may not be a float type.  Autocomplete on property definition p to see options and values.